This application is a national phase of international application PCT/JP98/03010 filed Jul. 3, 1998 which designated the U.S.
The present invention relates to a nonlinear optical fiber for generating a nonlinear optical phenomenon with respect to input light, an optical fiber coil comprising the nonlinear optical fiber, and a wavelength converter comprising the nonlinear optical fiber or optical fiber coil as its component.
It has been known in general that, when light having a high intensity (high optical density) propagates through a medium, there occur various nonlinear optical phenomena due to the nonlinear polarization in the medium. For example, among these nonlinear optical phenomena, second-harmonic generation is a phenomenon caused by a second-order nonlinear effect, in which, when two photons having the same energy enter the medium, a new single photon having a doubled energy is generated. On the other hand, four-wave mixing is a phenomenon caused by a third-order nonlinear effect, in which, when three photons enter the medium, a new single photon is generated therefrom. These nonlinear optical phenomena occur with the highest efficiency when both energy conservation law and momentum conservation law hold true among the plurality of photons involved in the nonlinear optical phenomena.
Applications of wavelength conversion and the like using such nonlinear optical phenomena occurring in optical fibers have also been reported. For example, a first document of M. J. Homes, et al., IEEE Photon. Technol. Lett., Vol. 7 (1995) No. 9, pp. 1045-1047 reports a nonlinear optical fiber obtained when a dispersion-shifted optical fiber is doped with Ge (germanium) element. A second document of K. Inoue, et al., IEEE Photon. Technol. Lett., Vol. 4 (1992) No. 1, pp. 69-72 and a third document of K. Inoue, et al., Opt. Lett., Vol. 19 (1994) No. 16, pp. 1189-1191 report attempts at wavelength conversion using the four-wave mixing generated in a dispersion-shifted optical fiber.
In addition to the above-mentioned first to third documents, a fourth document of D. A. Pastel et al., OFC"" 97 Technical Digest WL6b (1997) reports applications of nonlinear optical fibers and optical switches.
Further, in addition to the above-mentioned second and third documents, a fifth document of K. Inoue, IEEE Photon. Technol. Lett., Vol. 6 (1994) No. 12, pp. 1451-1453 reports a technique using the four-wave mixing in a dispersion-shifted optical fiber, which is a wavelength conversion technique in a wide wavelength band in which two excitation light beams having wavelengths different from each other are made incident on the optical fiber, and the wavelength of one of the excitation light beams is changed. Also, a sixth document of R. Ludwig, et al., Fiber and Integrated Optics, Vol. 15 (1996) pp. 211-223 reports an attempt at wavelength conversion using the four-wave mixing generated in a semiconductor amplifier.
Having studied the above-mentioned prior art, the inventors have found the following problems. Namely, in general, nonlinear optical phenomena occur weakly, with the third-order nonlinear optical phenomena being weaker than the second-order nonlinear optical phenomena, and therefore in any of the above-mentioned techniques it is necessary for the optical fiber to be elongated in order to attain nonlinear light (a new light component outputted as a result of a nonlinear optical phenomenon) having a sufficient intensity. In particular, for realizing a wavelength converter using the four-wave mixing generated in an optical fiber, the optical fiber for generating the nonlinear optical phenomenon is required to have a length of a few km or longer.
On the other hand, for realizing a wavelength converter or optical switch using a nonlinear optical phenomenon occurring in such an optical fiber, it is important technical problem to reduce the size of a coil constituting the optical fiber (hereinafter referred to as optical fiber coil). However, since the optical fiber is long as mentioned above, and its bending loss is large, it has been difficult for such an optical fiber coil to have a smaller size.
Meanwhile, the wavelength conversion technique disclosed in the above-mentioned sixth document is advantageous in that the apparatus itself can easily be made smaller since the four-wave mixing is generated in the semiconductor amplifier, and in that the band capable of wavelength conversion is wide, for example. Nevertheless, it has had a problem that noise is so high that the S/N ratio is low. By contrast, each of the wavelength conversion techniques disclosed in the second, third, and fifth documents is superior in terms of the S/N ratio since the four-wave mixing is generated in an optical fiber. Even in such a configuration, however, in order to attain converted light (a new light component generated by the wavelength conversion using the four-wave mixing) having a sufficient power, it is necessary for the optical fiber to have a length of a few km or longer, thus making it difficult for the optical fiber coil to reduce its size.
Also, due to the principle of the wavelength conversion using the four-wave mixing, the wavelength conversion efficiency is maximized when the excitation light wavelength is made to coincide with the zero-dispersion wavelength of the optical fiber, whereas the converted light drastically lowers its power as the difference between their wavelengths is greater. Hence, in the technique disclosed in the second document, the excitation light has a fixed wavelength, thereby the wavelength of the converted light is determined uniquely according to the signal light wavelength. Also, while the wavelength of the excitation light is made variable in the technique disclosed in the fifth document, the power of the converted light to be outputted would decrease as the wavelength shifts from the zero-dispersion wavelength. Namely, it has been very difficult to attain a wider band of wavelength conversion in the conventional wavelength converters using optical fibers.
In order to overcome the problems mentioned above, it is an object of the present invention to provide a nonlinear optical fiber which can generate a nonlinear optical phenomenon with high efficiency; an optical fiber coil, which can be made smaller, comprising the nonlinear optical fiber; and a wavelength converter with a compact configuration, which can output converted light having a desirable wavelength with high efficiency over a wide band, comprising the optical fiber coil or nonlinear optical fiber.
The nonlinear optical fiber according to the present invention is an optical fiber which generates a nonlinear phenomenon with respect to input light having a predetermined wavelength, e.g., one or more signal light components in the wavelength band of 1.55 xcexcm (1500 nm to 1600 nm), comprises a core region and a cladding region provided at an outer periphery of the core region, and is mainly composed of SiO2. In order to realize a wavelength conversion technique for overcoming the above-mentioned problems, this nonlinear optical fiber has, as characteristics with respect to the input light, a mode field diameter (hereinafter referred to as MFD) of 5 xcexcm or less, a polarization mode dispersion of 1 ps/kmxc2xd or less, a zero-dispersion wavelength of not less than 1.5 xcexcm but not greater than 1.6 xcexcm, a cutoff wavelength of not less than 1.4 xcexcm but not greater than 1.7 xcexcm at a fiber length of 2 m, a transmission loss of 3 dB/km or less, and a nonlinear coefficient of at least 10/W/km.
Also, in the nonlinear optical fiber according to the present invention, at least the above-mentioned core region is doped with GeO2 of not less than 15 mol % but not greater than 35 mol % on average, so as to realize a desirable refractive index profile. Thus, with respect to signal light in a desirable wavelength band (light in the 1.55-xcexcm band having often been used recently), this nonlinear optical fiber not only generates a nonlinear phenomenon with high efficiency but also is effective in that favorable signal light transmission characteristics can be secured, for example. In other words, this nonlinear optical fiber can yield converted light having a practically sufficient power at a shorter length.
More specifically, the nonlinear optical fiber has an MFD of 5 xcexcm or less. In general, the smaller is MFD, the better becomes the nonlinear characteristic of an optical fiber. In a silica glass type optical fiber in which at least its core region is doped with GeO2, the nonlinear coefficient can be made 10/W/km or higher when the above-mentioned condition for MFD is satisfied.
Also, the nonlinear coefficient of the nonlinear optical fiber according to the present invention is 10/W/km or more. It has been known that the wavelength conversion efficiency due to the four-wave mixing used for wavelength conversion is in proportion to the square of the nonlinear coefficient. As can also be seen from this fact, this nonlinear optical fiber would realize a wavelength conversion efficiency at least 10 times that of the conventional optical fiber, as long as the above-mentioned condition concerning the nonlinear coefficient is satisfied.
The polarization mode dispersion of the nonlinear optical fiber according to the present invention is not greater than 1 ps/kmxc2xd. The transmission rate to be currently realized is 10 Gb/s or higher, thus necessitating a pulse width of less than several tens of ps. In such a transmission system, it is necessary for the spreading of pulse in the optical fiber, which acts as an optical transmission medium, to be restricted to such an extent as to be negligible in practice, and therefore it will be more preferable if the above-mentioned condition concerning polarization mode dispersion is satisfied.
The zero-dispersion wavelength of the nonlinear optical fiber according to the present invention is not less than 1.5 xcexcm but not greater than 1.6 xcexcm. In the four-wave mixing, it is important for the zero-dispersion wavelength and the wavelength of the excitation light to coincide with each other. Also, when the zero-dispersion wavelength resides within this range, the excitation can be caused by the excitation light in a center portion of its wavelength conversion band.
The transmission loss of the nonlinear optical fiber according to the present invention is not greater than 3 dB/km. When the transmission loss within the optical fiber exceeds 3 dB/km, the effect of enhancing the nonlinear coefficient cannot effectively be utilized anymore. In other words, unless the above-mentioned condition concerning the transmission loss is satisfied, the wavelength conversion efficiency will deteriorate due to attenuation of the excitation light, and the power of the converted light will decrease.
Further, the cutoff wavelength of the nonlinear optical fiber according to the present invention, measured at an optical fiber length of 2 m, is not less than 1.4 xcexcm but not greater than 1.7 xcexcm. This condition concerning the cutoff wavelength is a condition measured at a fiber length of 2 m which is based on the international standard of ITU-T. In order for an optical fiber having a length of 1000 m or more to attain a cutoff wavelength of 1.55 xcexcm or shorter, the cutoff wavelength at a fiber length of 2 m must be 1.7 xcexcm or shorter. On the other hand, for satisfying the above-mentioned condition concerning the zero-dispersion wavelength, it is necessary for the cutoff wavelength at a fiber length of 2 m to be 1.4 xcexcm or longer.
The nonlinear optical fiber according to the present invention may be a polarization-maintaining optical fiber having a fixed plane of polarization. Such a polarization-maintaining optical fiber comprises a stress-applying structure for applying a stress in a direction substantially perpendicular to the light propagating axis of the optical fiber. Specifically, this stress-applying structure is positioned at locations in the cladding region substantially symmetrical to each other with respect to the core region, and may be realized by providing an SiO2 area doped with B2O3. This structure can hold the plane of polarization of input light at a predetermined direction and is further suitable for generating the nonlinear optical phenomenon with high efficiency.
For preventing a further nonlinear optical phenomenon from occurring between the input light and converted light, the dispersion slope of the nonlinear optical fiber according to the present invention is preferably not less than 0.01 ps/km/nm2, and more preferably not less than 0.05 ps/km/nm2, with respect to the input light (e.g., one or more light components each having a wavelength of 1500 nm to 1600 nm). As mentioned above, in the wavelength conversion using four-wave mixing, highly efficient wavelength conversion can be realized in the widest wavelength band when the wavelength of the input excitation light coincides with the zero-dispersion wavelength of the nonlinear optical fiber. When the optical fiber generating the nonlinear optical phenomenon has a small dispersion slope, however, wavelength conversion may occur efficiently between signal light beams in wavelength-division multiplexing (WDM) and between converted light beams being newly produced, thereby generating a very large number of converted light beams. Namely, all the light components other than the light component generated upon wavelength conversion from the incident excitation light are noise light components, which cause transmission quality to deteriorate. For avoiding such an inconvenience, the dispersion slope of the optical fiber generating the nonlinear optical effect is required to be set to a value which is somewhat high. Specifically, a nonlinear optical fiber having a dispersion slope of 0.01 ps/nm2/km or more is necessary for a 16-wave WDM transmission system which is ready for a signal channel interval of 100 GHz in compliance with the international standard of ITU-T, and a nonlinear optical fiber having a dispersion slope of 0.05 ps/nm2/km or more is necessary for a WDM transmission system of about 32 waves.
The nonlinear optical fiber according to the present invention may further comprise a structure for suppressing backward scattering caused by stimulated Brillouin scattering which may be generated by input light. Specifically, this scattering-light-suppressing structure can be realized when the GeO2 content in the core region is controlled such as to change continuously or stepwise along a longitudinal direction of the nonlinear optical fiber. This scattering-light-suppressing structure can be realized not only by controlling the GeO2 content but also by continuously changing the outside diameter of the core region along the longitudinal direction of the nonlinear optical fiber. In this case, the outside diameter of the cladding region may be either held constant regardless of the change in outside diameter of the core region or changed along with such change. Here, it is not necessary for the ratio of the outside diameter of the core region to the outside diameter of the cladding region to be constant.
The cladding region may include an area doped with F element. This configuration can also realize a refractive index profile of a depressed cladding structure. Similarly, the refractive index profile of the core region can employ various configurations.
Preferably, the nonlinear optical fiber according to the present invention comprises a hermetic coat disposed at an outer periphery of the cladding region. When the outer periphery of the cladding region is provided with the hermetic coat, then, even when the nonlinear optical fiber is submerged in water or exposed to a highly humid environment for a long period of time, fatigue will be restrained from progressing, and hydrogen will effectively be inhibited from invading the optical fiber. Thus, its reliability can be assured over a longer period of time.
The nonlinear optical fiber according to the present invention can be employed in an optical fiber coil which is suitable for a component such as a wavelength converter, an optical switch, or the like using a wavelength conversion technique. The optical fiber coil can be obtained when a nonlinear optical fiber having the above-mentioned characteristics including a predetermined polarization mode dispersion such as that mentioned above or a polarization-maintaining fiber (included in the nonlinear optical fiber according to the present invention) having the above-mentioned characteristics other than the predetermined polarization mode dispersion is wound at a predetermined diameter. In this case, these optical fibers can be made into coils by winding them around a bobbin having a barrel of a predetermined diameter.
In particular, for the purpose of an optical fiber coil according to the present invention, the nonlinear optical fiber preferably has, as characteristics with respect to input light having a predetermined wavelength, a bending loss of 0.1 dB/km or less and a polarization mode dispersion of 1 ps/kmxc2xd or less when wound into a coil having a minimum diameter of 60 mm or less. Also, as a configuration enabling a further smaller size in the optical fiber coil according to the present invention, the nonlinear optical fiber preferably has, as characteristics with respect to input light having a predetermined wavelength, a bending loss of 1 dB/km or less and a polarization mode dispersion of 2 ps/kmxc2xd or less when wound into a coil having a minimum inner diameter of 20 mm or less.
For preventing the transmission characteristic from deteriorating, the bending loss is preferably held as small as possible. In the case of a practical optical component, the maximum permissible value of bending loss is 1 dB/km. When the bending loss is greater than this value, output power may vary among wavelengths, thereby deteriorating the transmission characteristic remarkably. In the optical transmission at a frequency of 10 Gbit/sec or higher, there may be a case where the bending loss at 1 dB/km is still unfavorable. In this case, it is necessary for the coil diameter (the minimum diameter when the nonlinear optical fiber is wound, which can be defined by the barrel diameter of the bobbin) to be made greater, so as to suppress the bending loss to 0.1 dB/km or less.
The optical fiber coil according to the present invention can be realized by the nonlinear optical fiber having a length of 1 km or less. Here, the nonlinear optical fiber constituting the optical fiber coil (the nonlinear optical fiber according to the present invention, which generates a nonlinear optical phenomenon with high efficiency) may also be provided with a hermetic coat around the outer periphery of the cladding region.
The wavelength converter according to the present invention comprises a nonlinear optical fiber having the above-mentioned characteristics including a predetermined polarization mode dispersion such as that mentioned above or a polarization-maintaining fiber (included in the nonlinear optical fiber according to the present invention) having the above-mentioned characteristics other than the predetermined polarization mode dispersion. As the wavelength converter comprises the above-mentioned optical fiber coil as its component, it can attain a smaller size.
Specifically, the wavelength converter according to the present invention comprises an excitation light source for outputting excitation light; a multiplexing section for multiplexing and outputting the excitation light and signal light; a nonlinear optical fiber, having a length of 1 km or less, for receiving the excitation light and signal light outputted from the multiplexing section and generating a nonlinear phenomenon with respect to the excitation light; and a wavelength-converting section for selecting a wavelength of light to be outputted and outputting converted light having thus selected wavelength, the converted light being generated by the nonlinear optical fiber.
The nonlinear optical fiber (including the polarization-maintaining optical fiber) applicable to the wavelength converter according to the present invention has, as characteristics with respect to the excitation light, a mode field diameter of 5 xcexcm or less, a polarization mode dispersion of 1 ps/kmxc2xd or less or a fixed plane of polarization, a zero-dispersion wavelength of not less than 1.5 xcexcm but not greater than 1.6 xcexcm, a cutoff wavelength of not less than 1.4 xcexcm but not greater than 1.7 xcexcm at a fiber length of 2 m, a transmission loss of 3 dB/km or less, and a nonlinear coefficient of 10/W/km or more as mentioned above.
The nonlinear optical fiber applicable to the wavelength converter according to the present invention may be a polarization-maintaining fiber which maintains the plane of polarization of the excitation light at a predetermined direction. For generating a nonlinear optical phenomenon with high efficiency, it is preferable for the nonlinear optical fiber to have a dispersion value of approximately 0 ps/nm/km with respect to a predetermined wavelength component of the excitation light at a given point along its longitudinal direction.
Preferably, the excitation light source has a structure for changing the wavelength of the excitation light. In this case, though the wavelength of the converted light to be outputted from the wavelength converter becomes variable, high-power converted light can be obtained over a wide wavelength range, since the nonlinear optical fiber not only generates a nonlinear optical phenomenon with high efficiency but also has a short fiber length. Also, from the viewpoint of reducing the size of the apparatus as a whole, the excitation light source is preferably an optical fiber laser light source using an optical fiber doped with a rare earth element. For yielding converted light having a sufficient power, the excitation light source preferably outputs excitation light having a power of 10 dBm or higher. It is due to the fact that, when the converted light outputted from the nonlinear optical fiber has a power of xe2x88x9225 dBm or less, then its transmission characteristic would deteriorate under the influence of noise light even if it is amplified. Namely, for yielding converted light having a power of xe2x88x9225 dBm or higher, it is necessary for the excitation light source to output excitation light having a power of 10 dBm or higher.
In the wavelength converter according to the present invention, the input end of the nonlinear optical fiber is connected to the output end of the multiplexing section via an optical fiber whose connection loss is 1 dB or less. On the other hand, the output end of the nonlinear optical fiber is connected to the input end of the wavelength-selecting section via an optical fiber whose connection loss is 1 dB or less. When the excitation light and signal light entering the nonlinear optical fiber are attenuated at connecting portions, then the power of input light in the nonlinear optical fiber will decrease, thereby the resulting converted light will have a relatively smaller power. On the other hand, on the exit end side of the nonlinear optical fiber, the power of thus generated converted light as output light is also attenuated at the connecting portion with the wavelength-selecting section. Hence, for yielding converted light having a power of xe2x88x9225 dBm or higher, the connection loss at each of the above-mentioned connecting portions is required to be suppressed to 1 dB or less.
Preferably, in the wavelength converter according to the present invention, the input end of the nonlinear optical fiber is connected to the output end of the multiplexing section via an optical fiber having a mode field diameter successively increasing along a path though which the signal light propagates, whereas the output end of the nonlinear optical fiber is connected to the input end of the wavelength-selecting section via an optical fiber having a mode field diameter successively decreasing along the path though which the signal light propagates. Here, the optical fiber can be constituted by a plurality of optical fibers having mode field diameters different from each other. This configuration can also suppress the transmission loss in signal light, excitation light, and converted light, thereby yielding converted light with a sufficient power.
Further, in a preferable embodiment, the wavelength converter according to the present invention may comprise a converted-light-amplifying section for optically amplifying the converted light, and may comprise an excitation-light-amplifying section for optically amplifying the excitation light. This configuration can also eliminate detrimental effects caused by the transmission loss of each kind of light.
In thus configured wavelength converter, the wavelength band of signal light in which the decrease in spectral intensity of the converted light with respect to the maximum spectral intensity of the converted light becomes 3 dB or less has a width of 10 nm greater. Namely, a bandwidth of 10 nm or wider is necessary for a WDM transmission system of 16 waves or more which is ready for the signal channel interval of 100 GHz in compliance with the international standard of ITU-T. Also, since the permissible deviation in power of the resulting converted light is considered to be about 3 dB at the maximum, the wavelength bandwidth in which the wavelength conversion efficiency of the nonlinear optical fiber applied to the wavelength converter decreases by 3 dB is required to be 10 nm or wider.
In the wavelength converter according to the present invention, the nonlinear optical fiber outputs converted light at a conversion efficiency of 0.1% or higher with respect to the inputted signal light. Namely, it is necessary for the converted light to have a power of xe2x88x9225 dBm or greater as mentioned above. Hence, for obtaining such converted light, it is necessary for the wavelength converter to have a wavelength conversion efficiency of xe2x88x9230 dB (=0.1%) or higher.